OK I get that smiling does not come naturally to you, some theorize it is a mirror neuron difference in the brain that does that. And there are plenty of NTs who maybe smiling on the outside at work and grumpy on the inside, they just know a smile works wonders, well most of the time anyhow.
As you know in the NT world smiling is social validation and eases communications. You could try smiling at work as an experiment and see if people treat you different. When you have to tell that driver or customer that something is not going to arrive on time, try smiling as you say it, even if you are on the phone.
And having a twenty year old tell you 'you have to change that'. Well my automatic reaction would be 'stuff it kid' too. However he is young enough and foolish enough to be honest with you while the other adults will not bother.
If it is completely impossible for you to smile, then you cannot. But you might want to give it a try, as a scientific experiment.
From wikipedia on 'the smile'
“Service with a Smile”---This has always been at the core of American businesses since the beginning of the 1900s. Research continually proves that this is true; smiling really does increase attractiveness and likability between humans. In fact, people who tend to smile produce what is called the halo effect, which correlates with greater trust, greater financial earnings, and increased interpersonal cooperation.[5] In a time of increased stress due to cutbacks, high debt, and increasing family issues, employees are often required to work with a distressed public. However, a smile tends to convey respect, patience, empathy, hospitality and compassion. For example, when an employee smiles at a stressed customer, and exhibits excellent listening skills, most of the time, there is a report of total customer satisfaction.[6] Research also reports that people receive more help when they smile. Even the smile of a stranger produces more “Good Samaritan” effects on the receiver.[7] When you smile, even memory retrieval of your name is enhanced as is shown in neuroscience research, versus people who have neutral facial expressions.[8]